B. Carol Johnson

Simple Spectral Stray Light Correction Method for Array Spectroradiometers

A simple, practical method has been developed to correct a spectroradiometers response for measurement errors arising from the instruments spectral stray light. By characterizing the instruments response to a set of monochromatic laser sources that cover the instruments spectral range, one obtains a spectral stray light signal distribution matrix that quantifies the magnitude of the spectral stray light signal within the instrument. By use of these data, a spectral stray light correction matrix is derived and the instruments response can be corrected with a simple matrix multiplication. The method has been implemented and validated with a commercial CCD-array spectrograph. Spectral stray light errors after the correction was applied were reduced by 1-2 orders of magnitude to a level of approximately 10(-5) for a broadband source measurement, equivalent to less than one count of the 15-bit-resolution instrument. This method is fast enough to be integrated into an instruments software to perform real-time corrections with minimal effect on acquisition speed. Using instruments that have been corrected for spectral stray light, we expect significant reductions in overall measurement uncertainties in many applications in which spectrometers are commonly used, including radiometry, colorimetry, photometry, and biotechnology.

Overview of the Radiometric Calibration of MOBY

The Marine Optical Buoy (MOBY) provides values of water- leaving radiance for the calibration and validation of satellite ocean color instruments. Located in clear, deep ocean waters near the Hawaiian Island of Lanai, MOBY measures the upwelling radiance and downwelling irradiance at three levels below the ocean surface plus the incident solar irradiance just above the surface. The radiance standards for MOBY are two integrating spheres with calibrations based on standards traceable to the National Institute of Standards and Technology (NIST). For irradiance, the MOBY project uses standard lamps that are routinely calibrated at NIST. Wavelength calibrations are conducted with a series of emission lines observed from a set of low pressure lamps. Each MOBY instrument views these standards before and after its deployment to provide system responses (calibration coefficients). During each deployment, the stability of the MOBY spectrographs and internal optics are monitored using three internal reference sources. In addition, the collection optics for the instrument are cleaned and checked on a monthly basis while the buoy is deployed. Divers place lamps over the optics before and after each cleaning to monitor changes at the system level. As a hyperspectral instrument, MOBY uses absorption lines in the solar spectrum to monitor its wavelength stability. When logistically feasible during each deployment, coincident measurements are made with the predecessor buoy before that buoys recovery. Measurements of the underwater light fields from the deployment vessel are compared with those from the buoy. Based on this set of absolute calibrations and the suite of stability reference measurements, a calibration history is created for each buoy. These calibration histories link the measurement time series from the set of MOBY buoys. In general, the differences between the pre- and post-deployment radiance calibrations of the buoys range from +1% to -6% with a definitive bias to a negative difference for the post- deployment values. This trend is to be expected after a deployment of 3 months. To date, only the pre-deployment calibration measurements have been used to adjust the system responses for the MOBY time series. Based on these results, the estimated radiometric uncertainty for MOBY in-water ocean color measurements is estimated to be about 4% to 8% (kequals1). As part of a collaboration with NIST, annual radiometric comparisons are made at the MOBY calibration facility. NIST personnel use transfer radiometers and integrating spheres to validate (verify) the accuracy of the MOBY calibration sources. Recently, we began a study of the stray light contribution to the radiometric uncertainty in the MOBY systems. A complete reprocessing of the MOBY data set, including the changes within each MOBY deployment, will commence upon the completion of the stray light characterization, which is scheduled for the fall of 2001. It is anticipated that this reprocessing will reduce the overall radiometric uncertainty to less than 5% (kequals1).

Stray-Light Correction Algorithm for Spectrographs

In this paper, we describe an algorithm to correct a spectrographs response for stray light. Two recursion relations are developed:?one to correct the system response when measuring broad-band calibration sources, and a second to correct the response when measuring sources of unknown radiance. The algorithm requires a detailed understanding of the effect of stray light in the spectrograph on the instruments response. Using tunable laser sources, a dual spectrograph instrument designed to measure the up-welling radiance in the ocean was characterized for stray light. A?stray-light correction algorithm was developed, based on the results of these measurements. The instruments response was corrected for stray light, and the effects on measured up-welling in-water radiance were evaluated.

The Calibration and Characterization of Earth Remote Sensing and Environmental Monitoring Instruments. Chapter 10

The use of remote sensing instruments on orbiting satellite platforms in the study of Earth Science and environmental monitoring was officially inaugurated with the April 1, 1960 launch of the Television Infrared Observation Satellite (TIROS) [1]. The first TIROS accommodated two television cameras and operated for only 78 days. However, the TIROS program, in providing in excess of 22,000 pictures of the Earth, achieved its primary goal of providing Earth images from a satellite platform to aid in identifying and monitoring meteorological processes. This marked the beginning of what is now over four decades of Earth observations from satellite platforms. reflected and emitted radiation from the Earth using instruments on satellite platforms. These measurements are input to climate models, and the model results are analyzed in an effort to detect short and long-term changes and trends in the Earths climate and environment, to identify the cause of those changes, and to predict or influence future changes. Examples of short-term climate change events include the periodic appearance of the El Nino-Southern Oscillation (ENSO) in the tropical Pacific Ocean [2] and the spectacular eruption of Mount Pinatubo on the Philippine island of Luzon in 1991. Examples of long term climate change events, which are more subtle to detect, include the destruction of coral reefs, the disappearance of glaciers, and global warming. Climatic variability can be both large and small scale and can be caused by natural or anthropogenic processes. The periodic El Nino event is an example of a natural process which induces significant climatic variability over a wide range of the Earth. A classic example of a large scale anthropogenic influence on climate is the well-documented rapid increase of atmospheric carbon dioxide occurring since the beginning of the Industrial Revolution [3]. An example of the study of a small-scale anthropogenic influence in climate variability is the Atlanta Land-use Analysis Temperature and Air-quality (ATLANTA) project [4]. This project has found that the replacement of trees and vegetation with concrete and asphalt in Atlanta, Georgia, and its environs has created a microclimate capable of producing wind and thunderstorms. A key objective of climate research is to be able to distinguish the natural versus human roles in climate change and to clearly communicate those findings to those who shape and direct environmental policy.

A Method of Realizing Spectral Irradiance Based on an Absolute Cryogenic Radiometer

A technique is presented for realizing spectral irradiance using a large-area, high temperature, uniform, black-body source and filter-radiometers that are calibrated using a High Accuracy Cryogenic Radiometer. The method will be studied by calibrating irradiance lamps with this new technique and comparing the results with those obtained by the method currently employed at the National Institute of Standards and Technology (NIST). Progress to date and preliminary results are presented. The ultimate goal of the programme is to reduce the measurement uncertainties in the spectral irradiance scales that are made available to industry by calibrating deuterium and tungsten-halogen irradiance lamps.

Uncertainty Budgets for Realization of ITS‐90 by Radiation Thermometry

Recent international comparisons [1,2] and key comparisons have shown that the realization of the International Temperature Scale of 1990 (ITS‐90) above the freezing point of silver and its dissemination is more difficult than expected. In many cases, the deviations of the local scale realizations were larger than the combined estimated uncertainties could reasonably justify. On the other hand, it must be considered that the realization of the ITS‐90 by radiation thermometry is a complex exercise involving a large number of operations with many influencing parameters. Furthermore, the key comparisons need a unified approach to the treatment of uncertainties. Consequently, a rigorous standard approach for the calculation of uncertainties is necessary. In this paper three different operational schemes have been identified for realizing the ITS‐90 by radiation thermometry. For all three schemes an analysis is presented of the baseline parameters underlying the scale realization above the freezing point of si...

NIST activities in support of space-based radiometric remote sensing

We provide an historical overview of NIST research and development in radiometry for space-based remote sensing. The applications in this field can be generally divided into two areas: environmental and defense. In the environmental remote sensing area, NIST has had programs with agencies such as the National Aeronautical and Space Administration (NASA) and the National Oceanic and Atmospheric Administration (NOAA) to verify and improve traceability of the radiometric calibration of sensors that fly on board Earth-observing satellites. These produce data used in climate models and weather prediction. Over the years, the scope of activities has expanded from existing routine calibration services for artifacts such as lamps, diffusers, and filters, to development and off-site deployment of portable radiometers for radiance- and irradiance-scale intercomparisons. In the defense remote sensing area, NIST has had programs with agencies such as the Department of Defense (DOD) for support of calibration of small, low-level infrared sources in a low infrared background. These are used by the aerospace industry to simulate ballistic missiles in a cold space background. Activities have evolved from calibration of point-source cryogenic blackbodies at NIST to measurement of irradiance in off-site calibration chambers by a portable vacuum/cryogenic radiometer. Both areas of application required measurements on the cutting edge of what was technically feasible, thus compelling NIST to develop a state-of-the-art radiometric measurement infrastructure to meet the needs. This infrastructure has led to improved dissemination of the NIST spectroradiometric quantities.

Radiometric measurement comparisons using transfer radiometers in support of the calibration of NASA's Earth Observing System (EOS) sensors

EOS satellite instruments operating in the visible through the shortwave infrared wavelength regions (from 0.4 micrometer to 2.5 micrometer) are calibrated prior to flight for radiance response using integrating spheres at a number of instrument builder facilities. The traceability of the radiance produced by these spheres with respect to international standards is the responsibility of the instrument builder, and different calibration techniques are employed by those builders. The National Aeronautics and Space Administrations (NASAs) Earth Observing System (EOS) Project Science Office, realizing the importance of preflight calibration and cross-calibration, has sponsored a number of radiometric measurement comparisons, the main purpose of which is to validate the radiometric scale assigned to the integrating spheres by the instrument builders. This paper describes the radiometric measurement comparisons, the use of stable transfer radiometers to perform the measurements, and the measurement approaches and protocols used to validate integrating sphere radiances. Stable transfer radiometers from the National Institute of Standards and Technology, the University of Arizona Optical Sciences Center Remote Sensing Group, NASAs Goddard Space Flight Center, and the National Research Laboratory of Metrology in Japan, have participated in these comparisons. The approaches used in the comparisons include the measurement of multiple integrating sphere lamp levels, repeat measurements of select lamp levels, the use of the stable radiometers as external sphere monitors, and the rapid reporting of measurement results. Results from several comparisons are presented. The absolute radiometric calibration standard uncertainties required by the EOS satellite instruments are typically in the plus or minus 3% to plus or minus 5% range. Preliminary results reported during eleven radiometric measurement comparisons held between February 1995 and May 1998 have shown the radiance of integrating spheres agreed to within plus or minus 2.5% from the average at blue wavelengths and to within plus or minus 1.7% from the average at red and near infrared wavelengths. This level of agreement lends confidence in the use of the transfer radiometers in validating the radiance scales assigned by EOS instrument calibration facilities to their integrating sphere sources.

EOS AM-1 preflight radiometric measurement comparison using the Advanced Spaceborne Thermal Emission and Reflection radiometer (ASTER) visible/near-infrared integrating sphere

As a part of the Earth observing system (EOS) cross- calibration activities before the first flight (denoted AM- 1), a radiometric measurement comparison was held in February 1995 at the NEC Corporation in Yokohama, Japan, Researchers from the National Institute of Standards and Technology (NIST), the National Aeronautics and Space Administration/Goddard Space Flight Center (NASA/GSFC), the University of Arizona Optical Sciences Center, and the National Research Laboratory of Metrology (NRLM) used their portable radiometers to measure the spectral radiance of the advanced spaceborne thermal emission and reflection radiometer (ASTER) visible/near-infrared (VNIR) integrating sphere at three radiance levels. The levels each correspond to 83% of the maximum radiance that is expected to be measured using the three VNIR bands of the EOS ASTER instrument, which are centered at 0.56 micrometer, 0.66 micrometer, and 0.81 micrometer. These bands are referred to as bands 1, 2, and 3. The average of the measurements of the four radiometers was between 1% and approximately 1.5% higher for all three bands when compared to the NEC calibration of the sphere. A comparison of the measurements from the participating radiometers resulted in good agreement. These results are encouraging and will be followed by extension to other EOS AM-1 instrument calibration sources.

The Marine Optical BuoY (MOBY) Radiometric Calibration and Uncertainty Budget for Ocean Color Satellite Sensor Vicarious Calibration

For the past decade, the Marine Optical Buoy (MOBY), a radiometric buoy stationed in the waters off Lanai, Hawaii, has been the primary in-water oceanic observatory for the vicarious calibration of U. S. satellite ocean color sensors, including the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) and the Moderate Resolution Imaging Spectrometers (MODIS) instruments on the National Aeronautics and Space Administrations (NASAs) Terra and Aqua satellites. The MOBY vicarious calibration of these sensors supports international effort to develop a global, multi-year time series of consistently calibrated ocean color data products. A critical component of the MOBY program is establishing radiometric traceability to the International System of Units (SI) through standards provided by the U. S. National Institute of Standards and Technology (NIST). A detailed uncertainty budget is a core component of traceable metrology. We present the MOBY uncertainty budget for up-welling radiance and discuss additional considerations related to the water-leaving radiance uncertainty budget. Finally, we discuss approaches in new instrumentation to reduce the uncertainties in in situ water-leaving radiance measurements.